1. Field of the Invention
The present invention relates to differential amplifiers and bias circuits often used in conjunction with differential amplifiers. In particular, the present invention relates to providing a low-noise common mode feedback signal for adjusting common mode output voltage of a differential amplifier, and a bias current source which is immune to process variations.
2. Discussion of the Related Art
Differential amplifiers are building blocks of many analog circuits, especially analog amplifiers circuits in which high gain and high signal fidelity are required. An example of such an analog amplifier circuit can be a digital-to-analog conversion circuit suitable for use in computer graphics applications. To achieve the goals of high gain and high signal fidelity, differential amplifiers are required to have high common mode rejection, so as to provide the requisite dynamic output range.
Bias circuits are often used in conjunction with many applications of a differential amplifier. In an analog integrated circuit, for example, a bias circuit is often used to generate a reference current, which is then mirrored throughout the integrated circuit as a reference bias current wherever such a current is needed. However, variations in the manufacturing process of transistor current mirrors can result in significant variations in the mirrored current. For example, a 10% variation in the channel length of an MOS transistor in a current mirror can result in a more than 10% variation in the mirrored current.
Current sources are also extensively used with differential amplifiers. For example, a current source is frequently found in an input stage of a differential amplifier. Such a current source preferably has high noise-immunity and a high output impedance.